1. Field of the Invention
This invention relates generally to wireless communications and, more particularly, to the operation of a Radio Frequency (RF) transceiver within a component of a wireless communication system.
2. Description of the Related Art
The structure and operation of wireless communication systems are generally known. Examples of such wireless communication systems include cellular systems and wireless local area networks, among others. Equipment that is deployed in these communication systems is typically built to support standardized operations, i.e., operating standards. These operating standards prescribe particular carrier frequencies, modulation types, baud rates, physical layer frame structures, MAC layer operations, link layer operations, etc. By complying with these operating standards, equipment interoperability is achieved.
In a cellular system, a regulatory body typically licenses a frequency spectrum for a corresponding geographic area (service area) that is used by a licensed system operator to provide wireless service within the service area. Based upon the licensed spectrum and the operating standards employed for the service area, the system operator deploys a plurality of carrier frequencies (channels) within the frequency spectrum that support the subscribers' subscriber units within the service area. Typically, these channels are equally spaced across the licensed spectrum. The separation between adjacent carriers is defined by the operating standards and is selected to maximize the capacity supported within the licensed spectrum without excessive interference. In most cases, severe limitations are placed upon the amount of adjacent channel interference that may be caused by transmissions on a particular channel.
In cellular systems, a plurality of base stations is distributed across the service area. Each base station services wireless communications within a respective cell. Each cell may be further subdivided into a plurality of sectors. In many cellular systems, e.g., Global System for Mobile Communications (GSM) cellular systems, each base station supports forward link communications (from the base station to subscriber units) on a first set of carrier frequencies, and reverse link communications (from subscriber units to the base station) on a second set of carrier frequencies. The first set and second set of carrier frequencies supported by the base station are a subset of all of the carriers within the licensed frequency spectrum. In most, if not all, cellular systems, carrier frequencies are reused so that interference between base stations using the same carrier frequencies is minimized and system capacity is increased. Typically, base stations using the same carrier frequencies are geographically separated so that minimal interference results.
Both base stations and subscriber units include RF transceivers. Radio frequency transceivers service the wireless links between the base stations and subscriber units. The RF transmitter receives a baseband signal from a baseband processor, converts the baseband signal to an RF signal, and couples the RF signal to an antenna for transmission. In most RF transmitters, because of well-known limitations, the baseband signal is first converted to an Intermediate Frequency (IF) signal and then the IF signal is converted to the RF signal. Similarly, the RF receiver receives an RF signal, down converts it to IF and then to baseband. In other systems, the received RF is converted directly to baseband.
Radio receivers typically include several circuits that each provide an amount of gain to the received signals. For example, mixers and low pass filters each often provide gain. Because, however, the signal strength of a received signal can vary significantly, there is a need for amplifiers within the radio receiver whose gain level is adjustable. Programmable amplifiers often vary a feedback resistance value to adjust gain. Typically, a low noise amplifier is used to amplify the received signal prior to mixing it with a local oscillator. The level of amplification provided, however, must often be adjusted to compensate for fluctuations in received signal strengths.
FIG. 1A is a functional schematic diagram of a variable gain amplifier formed according to a known prior art design. The amplifier of FIG. 1A includes tuned circuitry with components selected to resonate at a specified frequency. Thus, as may be seen, the circuitry includes an inductive load 104 that is substantially coupled in series to a capacitive load 108. Further, a first resistive load 112 and a second resistive load 116 are coupled in parallel to inductive load 104. As may be seen, each of the resistive loads 112 and 116 are coupled in series to switches 124 and 128, respectively, for coupling or decoupling resistive loads 112 and 116 from being connected in parallel to inductive load 104.
Additionally, as may be seen, a MOSFET 132 is coupled to receive an input signal at its gate and is coupled in series with an additional isolation MOSFET 136 that is for providing isolation between MOSFET 132 and the load and tuning components described already. Thus, the use of isolation MOSFET 136 prevents MOSFET 132 from oscillating instead of amplifying a signal. MOSFET 136 is often referred to a Cascode device.
One drawback to the design of FIG. 1A is that the addition of resistive loads 112 and 116, especially if MOSFETs are used as resistors, is that the total capacitance of the circuit is modified each time a MOSFET resistor is turned on or turned off. Accordingly, if a circuit with inductive load 104 is tuned with capacitive load 108, the modification of the total network capacitance by the addition of new MOSFET resistors, or removal of MOSFET resistors, causes the tuning frequency to drift slightly. There is a need, therefore, for a circuit that enables resistive loads to be added or removed from the circuit to modify the circuit gain without affecting the oscillation frequencies or tuning of the circuit.
FIG. 1B is a functional schematic diagram of a differential prior art amplifier. As may be seen, the differential amplifier of FIG. 1B includes inductive loads 150A and 150B, isolation MOSFETs 154A and 154B, and a pair of amplification MOSFETs 158A and 158B. Isolation MOSFETs 154A and 154B have their gates coupled together so that current in one branch is reflected in the other branch. Amplification MOSFET 158A is for receiving a first portion of a signal input at its gate, while amplification MOSFET 158B is for receiving a second portion of a signal for amplification at its gate. The amplified signal produced by amplification MOSFET 158A results in a signal output at a node 162A, while the input of amplification MOSFET 158B results in an output signal being produced at a node 162B.
The operation of cascode differential amplifiers, such as shown in FIG. 1B, are known. The amplifier of FIG. 1B is shown to illustrate a typical configuration for the purposes of better explaining the described embodiments of the present invention. More specifically, the figures herein that describe the invention illustrate only one-half of a differential amplifier pair so as to simplify the explanation and make the operation more clear to understand. It is understood, however, that the invention described herein may readily be formed as a differential amplifier for amplifying positive and negative signals.
The inventors have realized, however, that adding or removing loading elements formed within integrated circuit radios (e.g., MOSFETs operating in a linear region as resistive loads) to change the amplifier loading changes the total network capacitance, thereby changing the circuit resonant frequency. Generally, removing or adding resistors at higher frequencies can affect the net capacitance of a circuit because parasitic capacitance is either removed or added when the loading is changed and therefore a circuit's frequency of oscillation (tuning) or resonant frequency changes. There is a need in the art, therefore, for a low power RF programmable amplifier that provides gain steps, as necessary, without changing a circuit resonant frequency.